Steel, and method of improving the creep strength thereof



' temperature.

Patented May 15, 1945 UNITED STATE S PATENT OFFICE STEEL, AND METHOD OF IMPROVING THE CREEP STRENGTH THEREOF I Richard F. Miller, Summit, N. 3., asslgnor to United States Steel Corporation corporation of Delaware No Drawing. Application August 14,

Serial No. 406,911

6 Claims.

This invention is specifically concerned with invention is concerned need not be specifically advanced, since they are well known. However, those unfamiliar with such steels may refer to pages 975 through 982 of The making, shaping and treating of steel, fifth edition, published by the Carnegie-Illinois Steel Corporation. It is to be understood that the present invention is not concerned with the high alloy steels such as those in the austenitic stainless series.

The present invention is based on the discovery that when a molybdenum containing low and medium alloy ferritic steel of the type designed for elevated temperature service is heated to above its omega solubility temperature and then cooled at a rate preventing visible omega precipitation, its creep strength can be adjusted to an optimum value by reheating the steel to a temperature below its omega solubility temperature and above the service operating temperature ofv the steel, and holding the steel at this secondnamed temperature for a time producing a faintly visible but indistinct omega precipitation, the steel then being cooled to prevent further -omega precipitation.

The omega phase results'fr'om .the molybcompletely soluble in austenite, but soluble in ferrite only to a limited degree. This solubility in ferrite appears to decrease with decrease of The omega phase difiuses and aeglomerates very slowly in ferrite-much more slowly than does-iron carbide. This diffusion may occur as a result of the migration of individual atoms, rather than by movement of whole molecules of the compound.

The omega solubility temperature for all the steels has not yet been ascertained, but it is known that it is exceeded when any 'of the steels are heated to fully austenitic condition. While air cooling generally provides a sufliciently fast cooling rate to prevent visible omega precipitation, microscopic examinations will definitely determine whether the rate chosen is sufilciently of Delaware, 2.

fast. The reheating temperature must be above the temperature at which the steel is to operate .when it is is placed in service, but it must not be so high that omega precipitation cannot occur. The holding time of the temperature chosen depends on the temperature and on the exact composition of the steel, since these determine the omega diffusing rate, but in practical work the steel can be heated progressively longer time periods, cooled andmicroscopically' examined, to determine what time period results in precipitation of the omega phase to an extent that is faintly visible but cannot be said to be a distinct precipitation. It is diflicult to describe exactly the appearance of the precipitate providing the maximum creepv strength, but creep strength tests may be used to determine the holding time providing the optimum creep strength.

As a specific example of the invention, a carbon-molybdenum steel of the type described, and

, which contained up to 20% carbon and about dnum contentof the steel and is believed to be this end.

.50% molybdenum, was heated to 1650 F. when it became entirelyaustenitic. to prevent visible omega precipitation and the steel was then subsequently reheated to about 1300 E, which is below the lower critical temperature of the steel but is above the temperature at which this steel operates in service. The holding time necessary at 1300 F. was five hours, this producing an omega precipitation that was just barely detectable by microscopic examination. Creep strength tests showed that. this fivehour period produced the optimum creep strength.

The creep strength of this steel after normalizing alone, or after normalizing and drawing in the conventional manner, was definitely inferior to the creep strength obtained by holding the temperature of the steel at 1300 F. for five hours. Microscopic examination failed to reveal a trace of the omega precipitate after normalizin or after drawing at 1200 F. in the conventio manner, or, in fact, even when the steel was hel at 1200 F.. for five hours. Prolongation of the 1300 F. holding period beyond the five hours produced quite a pronounced precipitate and an incidental lowering in the creep strength. It appears that there is an optimum omega particle size necessaryif maximum creep resistance is to be obtained and that the time and temperature of the treatment must be correlated to produce The precipitate produced by this treatment proved to be stable at operating temperatures as high as 1100 F., at which temperatures a speci- Air cooling served.

men or the carbon-molybdenum steel. subjected to the described heat treatment, maintained a linear creep rate for 14,000 hours, at which time the test was stopped. Microscopic examination failed to reveal any change in the particle size or the omega precipitate.

It was formerly common to normalize steels of the type to which this invention relates, and to subsequently temper or draw the steel at temperatures higher than the service operating temperature of the steel. This was done, however, solely to adjust the physical properties of the steel and with no thought of increasing its creep strength, due to the lack of any appreciation of the principles of the present invention. These prior art treatments could only accidentally result in a precipitate of the character produced in the case of the present invention. As a matter of fact, in no instance of this tempering or drawing,

whether performed for the purpose of adjusting the mechanical properties, or to stabilize the iron carbide, were those skilled in the art cognizant of the fact that such reheating involved the precipitype; said method comprising heating said steel to completely dissolve its phase resulting from its molybdenum content, cooling said steel at a rate preventing visible precipitation of said phase, reheating said steel to a temperature effecting precipitation of said phase and which is higher than the service operating temperature'of said steel, holding said steel at the second-named temperature to obtain a faintly visible but indistinct precipitation of said phase and then cooling said steel.

3. A method of improving the creep strength of molybdenum containing low and medium alloy territic steel of the type designed for elevated temperature service and containing up to carbon, from .40% to 2.00% molybdenum and being otherwise of a composition suitabl forsaid type; said method comprising heating said steel reheating 'said steel to a temperature effecting tation of a third phase. In most instances, these prior art treatments resulted in a decrease in the creep strength pfthe steel, whereas it has been shown the adjustment of the temperature and holding, time taught by the present invention provides the steel with its maximum creep strength.

In its broader form, this invention is applicable to any steel of the type described which has a composition resulting in a slowly. difiusing third phase that is soluble in austeniteand which is substantially insoluble in ferrite. This phase is possible in the case of such steels which contain tungsten, columbium, titanium, etc., with, of course, the carbon usual to its type. However, the invention is most applicable to molybdenum containing steels of the type in question, since the greatest creep strength i then obtained.

I claim:

1. A method of improving the creep strength of molybdenum containing low and medium alloy ferritic steel of the type designed for elevated temperature service, said method comprising heating said steel above its omega solubility temperature, cooling said steel at a rate preventing visible omega precipitation, reheating said steel to a temperature below said solubility temperature and above the service operating temperature of said steel, holding said steel at the second-named temperature, for a time producing a faintly visible but indistinct omega precipitation and then cooling said steel}.

2. A method of improving the creep strength of molybdenum containing low and medium alloy ferritic steel of vthe type designed for elevated temperature service and containing up to 20% carbon, from .40% to 2.00% molybdenum and being otherwise of a composition suitable for said precipitation of said phase and which is higher than the service operating temperature of said steel, holding said steel at the second-named temperature for a time effecting precipitation of said phase to a degree providing the optimum creep strength at said service operating temperature and then cooling said steel.

4. A molybdenum containin low and medium alloy ferritic steeLhaving a composition typical of the type designed for elevated temperature service and characterized by a structure including a faintly visible but not clearly distinct precipitation'of the phase resulting from its molybdenum content. 3 5. A low and medium alloy steel of the type designed for elevated temperature service and including an alloy of the class consisting of molybdenum, tungsten, columbium, and titanium, and characterized by a structure including a faintly visible but not clearly distinct precipitation of the phase resulting from its content of said class of alloy.

6. A method of improving the creep-strength of low and medium alloy ferritic steel of the type designed for elevated temperature service and including an alloy of the class consisting of molybdenum, tungsten, columbium, and titanium, said method comprising heating said. steel above its solubility temperature for the phase resulting from its content of said class of alloy, cooling said steel at a rate preventing visible precipitation of said phase, reheating said steel to a temperature below said solubility temperature and above the service operating temperature of said steel, and holding said steel at the second named temperature for a time producing a faintly visible but indistinct precipitation of said phase.

RICHARD-F. MILLER. 

